Quadrupole ion traps operated with radio-frequency (RF) potentials (also known as Paul traps) are used in mass spectrometry for accumulating ions and for ejecting pulsed packets of ions into a mass analyser. Suitable mass analysers include time-of-flight (TOF), electrostatic trap (EST), and Fourier Transform mass spectrometers (FT-MS). TOF mass spectrometers include linear TOF, reflectron TOF and multireflection TOF. EST mass spectrometers include orbital traps such as Kingdon traps, a type of which is marketed as ORBITRAP™ by the applicant and which utilises image current ion detection and Fourier Transform signal processing. FT-MS mass spectrometers include ORBITRAP™ mass analysers and ion cyclotron resonance mass analysers.
In many cases, the quadrupole ion trap must eject a packet of ions within a short time duration, the packet containing ions of a wide range of mass-to-charge ratios (m/z). The pulse duration should be uniformly small over the whole range of m/z.
In quadrupole ion traps the ions are confined by RF fields which are induced by the RF potentials which are applied to one or more trap electrodes. In 3D quadrupole ion traps one or more RF potentials are applied to one or more of a ring electrode and two end cap electrodes. Typically in linear quadrupole ion traps, four generally parallel rod electrodes have two opposite polarity RF waveforms applied, one to each pair of opposing rods.
Quadrupole ion traps for ejection to a mass spectrometer usually operate with a gas introduced into the trap volume, and collisions between ions and the gas molecules cause the ions to lose energy progressively with each collision and thereby cool to approximately the gas temperature, which may be room temperature, or lower in cryogenic traps, and the ions are said to be thermalized. This serves to reduce the spread in velocities in the direction of ejection, and hence reduce the range of times at which ions of the same m/z reach the mass spectrometer, and in some cases its detector. This range of times directly limits the mass resolving power of a TOF mass spectrometer, for example, and hence should be as small as possible.
Once the ions have undergone enough collisions with the gas to cool all the ions within the desired mass range sufficiently, the ions are ejected from the quadrupole ion trap. In the 3D quadrupole ion trap, ions are ejected through a small aperture in one of the end caps. In the linear ion trap, ions are ejected either from one end of the linear trap generally along its axis (axial ejection), or orthogonal to the trap axis through one of the gaps between the rod electrodes, or through a slot formed in one of the rod electrodes (orthogonal ejection). Orthogonal ejection is preferable because the ion packet is then smaller in the direction of ejection. To eject the ions, either an ejection potential is applied across the trap in addition to the RF trapping potentials, or the RF trapping potentials are turned off and an ejection potential is applied.
In some cases one or more RF trapping potentials are turned off when they reach a zero crossing point. As used herein in relation to applied RF potentials, the term “zero crossing point” refers to a time at which the (time-varying) RF potential is momentarily at zero potential, either during passage from a positive potential to a negative potential, or during passage from a negative potential to a positive potential. Where two RF potentials are applied to an ion trap, those potentials are typically at opposite phases from each other. Hence when one RF potential reaches a zero crossing point, so does the other RF potential, but one RF potential is passing from a positive potential to a negative potential and the other RF potential is passing from a negative potential to a positive potential.
Ejected ions are introduced into a mass analyser and travel within the analyser along an analyser flight path. Ions of different m/z travel the analyser flight path either traversing a distance to a detector in different times, or undergoing oscillatory motion within the analyser at different frequencies. The analyser flight path may be linear, comprise linear portions, or may be curved or comprise curved portions. In order to travel along the analyser flight path the ions must be injected into the analyser along an injection trajectory. As used herein the term “analyser injection trajectory” refers to the injection trajectory which ions must follow in order to enter the analyser so that they subsequently travel along the analyser flight path. It will be understood by the skilled person that the analyser injection trajectory and the analyser flight path are finite volumes of space within which ions travel though they may be represented as lines.
U.S. Pat. No. 5,569,917 describes the simultaneous application of opposite polarity extraction potentials of similar magnitude to the two end caps of a 3D quadrupole ion trap in order to eject ions in a collimated beam. The beam was then post-accelerated for use in a TOF mass spectrometer.
U.S. Pat. No. 6,380,666 describes the simultaneous application of opposite polarity extraction potentials of different magnitudes to the two end caps of a 3D quadrupole ion trap, without post-acceleration.
U.S. Pat. No. 6,483,244 describes a 3D quadrupole ion trap and an electronic arrangement with switches in which the RF trapping voltage is turned rapidly to zero and extraction voltages are applied to the end cap electrodes at nearly the same time as the RF potential is terminated. In this arrangement the RF trapping voltage may be terminated at any chosen part of the RF cycle by operation of the switches. On terminating the RF trapping potential, the RF trapping potential actually present on the ring electrode of the ion trap approaches zero with a time constant determined by the capacitance between the electrodes of the trap and the internal resistance of the switches. This time constant is small enough to prevent the ions escaping from the ion trapping region. However the problem of abrupt stopping the RF voltage in the moment of its maximal span still remains unresolved because of considerable capacitance of the trap's electrodes.
U.S. Pat. No. 7,250,600 describes a 3D quadrupole ion trap in which the RF trapping potential is terminated in a way which minimises the spatial spread of ions within the trap at the time the ejection potential is applied. The ions within the trap move under the influence of the RF field within the trap, moving from a larger volume of space within the trap to a smaller volume as a function of the phase of the RF potential applied to the trap ring electrode. The RF trapping potential is terminated at a time when ions of a given polarity are converging or have converged to the smaller volume and the ions are ejected from the trap from a smaller volume within the trap thereby minimising the variation in starting positions of the ejected ions. The RF trapping potential is terminated at a zero crossing point, i.e. at a time at which the time-varying potential is momentarily at zero potential. Due to the various electronic components connected to the trap, the RF potential could not, in this arrangement, be terminated instantaneously, and a time delay between the attempted termination of the RF potential and the application of the ejection pulse was provided. It is explained that during this time period the ions do not experience a trapping effect and may move freely and disperse, and having a large time delay is not recommended.
U.S. Pat. No. 7,256,397 describes a 3D quadrupole ion trap in which the RF trapping voltage applied to the ring electrode is terminated at a predetermined phase and an ejection potential is applied across the end cap electrodes after a predetermined time period, the predetermined phase and the predetermined time period being chosen such that the actual potential on the ring electrode is the same after the predetermined time period irrespective of the amplitude of the RF voltage when it is terminated. By this means a time at which the ejection potential is applied may be found so that the actual voltage on the ring electrode is the same regardless of the m/z range trapped (which is determined by the amplitude of the RF trapping potential applied) and the time delay during which no quadrupole field exists within the trap and in which ions may disperse is minimised.
US patent application 2014/0008533 describes a 3D quadrupole ion trap in which a single phase RF trapping voltage is applied to both end cap electrodes, and is switched down shortly before a zero crossing point at which the ion cloud spatially contracts. A DC extraction potential is then applied to at least one of the two end cap electrodes.
U.S. Pat. No. 5,763,878 describes a linear multipole ion trap with orthogonal ejection of ions. The multipole may be of various forms including hexapole, quadrupole and distorted quadrupole arrangements. For ion ejection the RF trapping potential is terminated at a zero crossing point and ejection potentials are applied to various electrodes to create an approximately uniform field within a portion of the trap.
U.S. Pat. Nos. 7,498,571 and 8,030,613 describe an electrical circuit including a switched shunt to short out a secondary winding of the RF voltage driver to rapidly switch off the RF trapping potential. A DC ejection potential may then be applied with or without a time delay for axial or orthogonal ejection from a linear quadrupole trap. The RF trapping potential is rapidly switched off at a zero crossing point.
When an extraction field Ex is applied to an ion trap, there is necessarily a variation in potential induced within the trap volume, there being a potential gradient in the direction of ejection for ions of a chosen polarity. Accordingly, ions at different spatial locations within the trap which are at different locations on the potential gradient will undergo differing potential changes on travelling to the entrance of the mass analyser. The spatial spread δx in the direction of the axis of extraction, x, within the ion trap, produces a kinetic energy spread when the ions arrive at the mass analyser, δK=q·Ex·δx, where q is the charge on the ions. As described above, prior art methods of ion extraction have given consideration to reducing the spatial spread of ions within the trap at the moment of ejection, notably as described in U.S. Pat. No. 7,250,600, and this reduces the kinetic energy spread of the ions which arrive at the mass analyser.
However, a temporal or time-of-flight focus may be formed, where ions which were farthest from the mass analyser at the moment the ion ejection field was applied undergo the largest potential drop and thus have the highest kinetic energy, subsequently overtaking ions which were closest to the mass analyser at the moment the ion ejection field was applied. A temporal focus may be formed to coincide with a desired location within a mass spectrometer, and may be imaged to another location, such as a detector plane in a TOF mass spectrometer, for example. Where a temporal focus is formed, the temporal spread of ions at the temporal focus is not dominated by the initial spatial spread δx in the direction of the axis of extraction, x, within the ion trap, but instead is predominantly determined by the initial velocity spread in the direction of the axis of extraction δvx of the ions in the trap.
Typically ions have a spread in velocities ranging from −δvx/2 to +δvx/2 at the moment the extraction field is applied. If a first ion has a velocity −δvx/2 it travels away from the mass spectrometer for a period of time, it takes a time δt=m·δvx/q·Ex to travel away, turn around and come back to its initial location. Meanwhile a second ion starting from the same position with velocity +δvx/2 has progressed towards the mass spectrometer. The time difference δt between these two ions cannot be compensated for in practice as the ions possess no characteristics by which they may be distinguished from one another, being of the same energy and originating from the same point, and at represents the dominant temporal spread of the ions at a temporal focus. The time difference δt is called the turn-around time (for obvious reasons). This temporal spread directly limits the mass resolving power which may be obtained by the mass spectrometer, according to tTOF/2.δt, for a TOF mass spectrometer, for example, where tTOF is the total time of flight from the ion starting point within the ejector to the detector of the spectrometer.
Hence where a temporal focus is formed, it is desirable not to extract ions in a way which minimises their spatial spread δx within the ion trap, as taught in some of the prior art noted above, but instead to minimise their velocity spread δvx within the trap at the moment of ejection.
It has been suggested in U.S. Pat. No. 7,897,916 that additional velocity spread may be induced in the ions if upon applying the extraction field the RF trapping field has not stabilised, and that it is important to rapidly terminate the RF trapping field to very low levels in order to minimise this effect. However as already discussed it is difficult practically to suppress the RF trapping field if it is terminated at any time other than when the RF potential is at a zero crossing point.
In a RF quadrupole ion trap containing a buffer gas, where the ions have been thermalized due to collisions with the gas molecules, the ion ensemble is known to oscillate in phase with the RF potential applied to the trap electrodes, for a wide range of m/z. Phase space volume is conserved and when the ions are confined to their minimum extent in one direction they possess their maximum velocity spread in that direction (the ion trajectories are crossing over one another). Conversely, when the ions are at their largest spatial extent in one direction, they possess the minimum velocity spread in that direction. In a linear quadrupole ion trap, when the RF potential on the x rods is at a maximum positive voltage, ions of a positive polarity are at their largest spatial extent in x and at this time the ions possess their minimum velocity spread in x. However whilst this is the most desirable moment at which to eject the ions, to provide the lowest velocity spread in the x direction, the RF potentials applied to the rods are at that moment at a maximum, which may be several thousand volts, and as already described, it is difficult practically to terminate rapidly the potentials on the rod electrodes when the voltages are at a maximum due to the capacitance of the trap electrodes.
European Patent 1302973 describes a 3D quadrupole ion trap in combination with an orthogonal ejector and a TOF mass spectrometer. Ions are ejected from the quadrupole ion trap which contains a buffer gas (sometimes called a collision gas) to cool the ions by multiple collisions, and the ions travel into a region of higher vacuum for subsequent orthogonal acceleration. A high acceleration potential is only applied to the orthogonal ejector, and this reduces the number of high energy collisions between the sample molecular ions and gas molecules, thereby reducing the dissociation of the sample ions. The m/z range of ions admitted to the mass spectrometer is limited by the spread of velocities in the direction of ejection from the trap, and two means for reducing the velocity spread of ions were described: (1) increasing the ejection field within the trap during the time of ejection; (2) varying an electric field in the region between the trap and the orthogonal ejector. Due to the use of an orthogonal extractor, the velocity spread in the direction of ejection from the trap does not affect the mass resolution of the TOF mass spectrometer, rather, the velocity spread in the direction of the time of flight in the spectrometer is a limiting factor. No means for limiting this were described.
U.S. Pat. No. 7,897,916 describes a linear quadrupole ion trap with orthogonal ejection of ions through a slit in one of the rod electrodes to a TOF mass analyser. In one embodiment the trap is interfaced directly to the TOF mass analyser; in another embodiment the trap supplies ions to an orthogonal ejector which sends ions into the TOF mass analyser. The ion trap is driven with a so-called “digital drive” in which the potentials applied to the electrodes are not sinusoidal, but are rapidly switched DC potentials, switched between negative and positive values with equal time for each value providing a square wave drive with 50% duty cycle. Immediately prior to ejection the time period of the switched square wave is increased and an extraction pulse is then applied shortly after. The trapping potentials may be arranged so that one phase is applied to one pair of opposing rod electrodes and the opposing phase is supplied to the other pair of opposing rod electrodes, or alternatively only one phase may be employed, connected to only one pair of opposing rod electrodes and the other pair of opposing rod electrodes are at 0V until an extraction pulse is applied to them. In the latter case, the switched trapping potential is continuously applied to the pair of rod electrodes during the ejection phase, only the switching time period is increased prior to ejection. Ejection of ions was matched to the phase for which the energy spread of ions in a desired direction was at a minimum. The desired direction was varied depending upon the embodiment: where ions were ejected directly from the trap to the TOF mass spectrometer, the desired direction was in the direction of ejection from the trap, as this was the direction of time-of-flight in the TOF mass spectrometer; where the ions were ejected from the trap to an orthogonal ejector the desired direction was orthogonal to the direction of ejection from the trap, to generally be aligned with the direction of time-of-flight in the TOF mass spectrometer. Due to the use of stepped DC trapping potentials, the electric field within the quadrupole ion trap was constant during the period of ion ejection, albeit at a high amplitude. However use of a square or rectangular waveform has practical difficulties, since it necessarily involves abrupt switching of large voltages very rapidly. Practical realization of this approach is difficult because any abrupt switching of the RF voltage involves re-charging of the capacitance formed by the trap's electrodes. Unlike the case of sinusoidal waveform in an RF tank, the electric energy stored in the capacitance in not recuperated by a magnetic coil but must be dissipated. Voltage ‘ringing’ also is very difficult to avoid.
In view of the above, the present invention has been made.